The present invention relates to filters, converters and amplifiers, and more particularly to a variable transconductance amplifier with improved linearity and that maximizes input voltage utilization.
The bipolar junction transistor (BJT) differential pair is one of the most important integrated circuit building blocks. It can be used as a transconductance stage for variable gain amplifiers and gm-C filters. The differential output current ID is related to the emitter bias current IE and the input differential voltage VDif in accordance with the following equation 1:
IDxcx9cIE tan h[VDif/2VT]xcx9c(IE/2VT)VDifxe2x80x83xe2x80x83(EQ 1)
where xe2x80x9cxcx9cxe2x80x9d denotes xe2x80x9capproximately equalxe2x80x9d and where VT is a thermal coefficient voltage (the voltage equivalent of temperature, where VT=kT/q, where xe2x80x9ckxe2x80x9d is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin [absolute scale], and xe2x80x9cqxe2x80x9d is the magnitude of the charge of an electron), and when VDif less than VT.
The transconductance (gm), which is the ratio of output current to input voltage, can be scaled by changing the emitter bias current. The only major problem with this stage is the maximum useful input voltage. Significant distortion occurs when the peak input voltage is in excess of about 2VT. Linearity can be improved by adding resistive or inductive emitter degeneration, but then the transconductance cannot be changed electronically.
Prior art approaches to increasing the allowable input voltage (over that of a single differential pair) include dividing the input voltage across series-connected differential pairs or using an attenuator in front of the differential pair(s). The attenuator usually has low input impedance and does not lend itself to electronic gain control. The series-connected differential pairs (multi-tanh) becomes impractical for large input voltages, where xe2x80x9ctanhxe2x80x9d refers to the hyperbolic tangent function.
A variable transconductance amplifier according to the present invention includes a variable attenuator stage coupled to a transconductance stage. The variable transconductance amplifier allows the overall transconductance to be decreased when the input signal is increased without distorting the output signal. Embodiments of the variable attenuator stage described herein exhibit a relatively high input impedance and are electronically controllable. Embodiments of the transconductance stage also have electronically controllable gain. In this manner, electronic gain control can be applied to either or both stages so that attenuation may be increased in the presence of increasing input voltage to maintain linearity. In various embodiments described herein, electronic gain control is achieved using electronically controllable current devices, such as current sinks or current sources, that may be controlled by external circuitry as is known to those skilled in the electronic arts.
In a particular embodiment, the variable attenuator includes first and second differential to single-ended transconductance stages, where each stage is biased by a respective one of first and second current devices. Also, each stage has a first control terminal, so that both stages collectively receive a differential input voltage signal. Each stage further has a second control terminal that develops a current signal. At least one reactive element or a reactive circuit is coupled between the second control terminals of the pair of differential to single-ended transconductance stages, so that the developed current signal flows through this element or circuit. In this embodiment, the transconductance stage includes a first differential pair having first and second control terminals and first and second output terminals, where the first and second control terminals of the first differential pair are coupled to the first and second control terminals, respectively, of the first differential to single-ended transconductance stage of the variable attenuator. The transconductance stage further includes a second differential pair having first and second control terminals and first and second output terminals, where the first and second control terminals of the second differential pair are coupled to the first and second terminals, respectively, of the second differential to single-ended transconductance stage of the variable attenuator. The first output terminal of the first differential pair is coupled to the second output terminal of the second differential pair and forms a first polarity of a differential output current signal. Also, the first output terminal of the second differential pair is coupled to the second output terminal of the first differential pair and forms a second polarity of the differential output current signal.
The particular configuration of the reactive element depends upon the specific application in which the variable transconductance amplifier is employed. In one embodiment, a single resistor may be used. In alternative embodiments, an inductor, a capacitor, or any combination of such components may be employed as appropriate for the specific application and signal frequency. For example, the combination of components may have a characteristic frequency or frequency response designed for specific applications.
The first and second current devices may be electronically controllable to adjust the gain of the variable attenuator and to adjust the maximum allowable input voltage of the differential input voltage while maintaining linearity. In a similar manner, the first and second differential pairs may each be biased by at least one additional current device that is electronically controllable to adjust the transconductance between the differential input voltage signal and the differential output current signal. In one embodiment, the first and second differential pairs of the transconductor stage may each have a bias terminal, where the bias terminals of the first and second differential pairs may be coupled together. Also, a third current device may be coupled to the common bias terminals of the first and second differential pairs. This third current device may be electronically controllable as the first and second current devices.
In one alternative embodiment of the variable attenuator, each differential to single-ended transconductance stage includes a differential pair of transistors including a diode-coupled transistor, a current sink and a current mirror coupled to the differential pair. In another alternative embodiment of the variable attenuator, each differential to single-ended transconductance stage includes a differential pair of transistors including a first transistor coupled to a voltage supply signal and a diode-coupled transistor. The first and second current devices are each current sinks coupled to a respective one of the differential pairs. Also, for each differential pair, a current source is coupled to the diode-coupled transistor, where the current source sources a current level that is proportional to the current developed by the corresponding current sink of the differential pair.
In yet another embodiment, first and second current sources are provided for each differential pair, where each is coupled to source current to a respective one of the transistors of the differential pair. In this latter configuration, the current sources each source a current that is proportional to the current developed by a corresponding current sink of the differential pair. In yet another alternative embodiment, the differential pairs of transistors of the first and second differential to single-ended transconductance stages are cross-coupled. Capacitors may be added in the cross-coupled configuration to facilitate operation at higher frequencies.
Many variations are possible for each of the variable attenuator and transconductance stages of the variable transconductance amplifier. For example, different types of transistors may be used, such as NPN or PNP bipolar junction transistors, metal-oxide semiconductor, field-effect transistors (MOSFETs), etc. The current mirrors, for example, may be configured using common base PNP bipolar junction transistors as known to those skilled in the art. Current mirrors, if used, may have a 1:1 current ration or any other desirable current ratio. The relative current levels between corresponding current sources and current sinks of each portion of each stage may be altered if desired for certain configurations. The transistors of each transistor pair or differential pair configuration may have different sizes, emitter areas, current capacities, etc. Level shifting elements may be added if desired, such as for configurations required to handle large input voltages that might otherwise drive certain transistors into saturation. For example, diodes may be connected in series with the bases of selected bipolar junction transistors. For high frequency operation, capacitors may be coupled between the each of the first and second differential to single-ended transconductance stages of the variable attenuator and the reactive element.